There are a number of U.S. patents that disclose electronic apparatus for printing indicia on labels, some of which are described in U.S. Pat. Nos. 4,440,248, Teraoka; 4,501,224, Shibayama; 4,630,538, Cushing; and 4,655,129, Wirth et al.
The electronic machines for printing labels of the type disclosed above all include the same general combination of elements, a print head, means for feeding labeling media to be printed past the print head, a microprocessor, a read only memory programmed with appropriate instructions to operate the microprocessor, a random access memory, a keyboard with letter, number, and function keys for the entry of alphanumeric information concerning the indicia to be printed, and a visual display such as a LED/LCD unit to assist the operator in using the machine.
The labeling media comprises a roll of pressure sensitive tape (continuous media or die cut labels)that is attached to a continuous roll of release liner. The labeling media is fed through the printer and legends are printed on the tape. Labels are formed using the continuous tape by cutting the media after the legends are printed thereon. The labels are then removed from the release liner and attached to the objects needing identification. As there are many types of label applications, there are many combinations of tape and release liners that provide labels of varying sizes, colors, formats, and environmental resistance.
A particular type of print head employs thermal transfer printing technology. Thermal transfer printing uses a heat generating print head to transfer a colored coating containing wax, carbon black, or the like, from a thermal transfer ribbon to a labeling media. By using digital technology, characters are formed by energizing a sequence of pixels on the print head which in turn melt the coating on the ribbon transferring the image to the labeling media.
In a known thermal transfer printer such as a label printer, labeling media is fed by a platen roller simultaneously with a ribbon feed roller feeding an ink transfer ribbon. While the labeling media driven by the platen roller runs between the print head and the rotating platen roller, the transfer ribbon is passed between the print head and the platen roller by rotating the ribbon feed roller. As a result, the labeling media and the transfer ribbon pass together in overlay relationship between the print head and the platen roller.
The labeling media is fed past the print head from a supply spool driven by a labeling media drive subassembly. The labeling media drive subassembly has a drive side driving the roller, and a free wheel side which is not driven. Typically, the labeling media is justified to the drive side within the printer, and the free wheel side is adjusted to the width of the media. Justifying the labeling media to one side within the printer causes the labeling media to be off center to that of the print head. This causes an upset at the printer dot line, and an uneven pressure loading of the print head resulting in poor print quality.
One solution to justifying the labeling media to one side is to adjust the drive side and the free wheel side such that the labeling media is centered on the print head. However, an adjustable drive mechanism is expensive.
Furthermore, typical printer spools require a locking mechanism to ensure proper drive system engagement. This complicates engaging and disengaging the spool from the printer drive systems. These drive systems are also subject to slipping (i.e., torque out) which can cause the printer to jam. Therefore, a need exists for an improved printer drive system and spool.